My H&R cylinder bore .410 is very accurate using Remington bulk pack Sluggers. They are, however, quite soft and fragment easily. Brenneke makes a very good slug, harder alloy and quite accurate as well. My gun doesn't like any 3" shells, so I stick with the Sluggers or Brenneke Close Encounter. BTW, I have had good luck with Federal Handgun 000 buckshot in 2 1/2" as well. Palm-sized groups at 15 yards are the norm. YMMV. I also have a 500E with a 24" full choke barrel that likes the Brenneke 3" slugs, but keyholes the 2 1/2" Close Encounters....you'll probably have to try several different ammos to find the best combo...
Scott B.